A conventional hermetic compressor (hereinafter referred to simply as “compressor”), which is formed of a compressing mechanism and an electric motor both accommodated in a housing hermetically welded, is disclosed in Japanese Patent Unexamined Publication No. H06-159274. This compressor is free from refrigerant leakage or water invasion, so that it has been widely used in air-conditioners or refrigerators because of its high reliability.
FIG. 7 shows a sectional view of a conventional compressor. In FIG. 7, compressing mechanism 303, electric motor 304 and refrigerating machine oil are accommodated in cylindrical housing 302 to form the compressor. Housing 302 is provided with discharging tube 306 at its upper end for discharging compressed gas refrigerant.
Compressing mechanism 303 is rigidly mounted to housing 302, and is coupled to sucking tube 305 which feeds the gas refrigerant thereto. Compressing mechanism 303 is coupled to motor 304 with driving shaft 307, so that motor 304 drives compressing mechanism 303.
Motor 304 is placed above compressing mechanism 303 and connected to hermetic terminal 308 welded to the upper end of housing 302. Terminal 308 is excellent in pressure resistance and airtight performance, and motor 304 is powered by an external source via terminal 308.
The foregoing compressor supplies the refrigerating machine oil pooled in housing 302 to compressing mechanism 303 and its bearings for lubrication. The refrigerating machine oil pooled in housing 302 is discharged together with compressed gas refrigerant from the compressor. Under normal conditions, the oil circulates through a refrigerant circuit and returns to the compressor, so that the amount of the oil is maintained in housing 302. However, the amount of the oil varies depending on the operation, and it sometimes becomes short and fails in lubrication.
The problem discussed above has been addressed by some proposals, for instance, Japanese Patent Unexamined Publication No. 2001-12351 discloses the following idea: Oil surface position 309 of the refrigerating machine oil in housing 302 is sensed by a sensor, so that a shortage of the oil is detected for protecting the compressor. In other words, a detection of a lower surface position 309 of the oil carries out a protecting action, such as stopping the compressor, or collecting the oil from the refrigerant circuit, thereby avoiding damage to the compressor.
Detection of oil surface 309 in housing 302 needs housing 302 to be equipped with sensors, and transmitting signals of the sensors to the outside of housing 302. For this purpose, a conventional compressor mounts sensors rigidly in housing 302, and provides housing 302 with terminals for transmitting the signals to the outside. However, this structure complicates the compressor, and the add-on terminals will invite a defect in the airtight performance. This structure also needs connecting the sensors to the terminals in housing 302, so that a possible disconnection will lower the reliability.
The foregoing publication (No. 2001-12351) also discloses that an oil surface sensor, which is integrally formed of a detector for detecting an oil surface in the housing and hermetic terminals, is mounted on a side-wall of the housing. However, since the side-wall shapes like a cylinder, the mounting of the sensor onto the side-wall will invite a defect in airtight performance due to distortion, or causes a failure in airtight performance due to a collision in assembling the compressor.
The detector is placed inward of the inside wall of the housing, so that the detector sometimes erroneously detects the oil surface. Because parts of the refrigerating machine oil touch the detector of the oil surface sensor when the oil discharged together with the gas refrigerant during the operation returns from a position higher than the oil surface to the lower section of housing 302, or when the oil after the lubrication through the compressor is discharged from the upper section and returns from a position higher than the oil surface to the lower section of housing 302.
Further, the oil surface sensor is mounted in the housing at a place corresponding to the lower limit of the oil surface, and after a detection of the lower limit of the oil surface, the oil surface cannot rebound immediately although an oil-surface rebounding action is taken. This delay further lowers the oil surface. This phenomenon sometimes causes serious damage to the compressor.